In order to identify a desired kind of articles when among various other kinds of articles in a manufacturing process, it is well known to use a method of identifying the hue or color of the articles or objects.
FIG. 1 shows an example of a conventional color sensor for recoginizing color of articles. Referring to FIG. 1, rays of white light are emitted from a light source 1 made of a tungsten lamp and a portion of the rays of light is reflected by a half mirror 2. The light reflected by the half mirror 2 is collimated by a lens 3 at a position at which an article or an object to be recognized is placed. Each of the objects has identifying a specific color given to it either by the color of the material forming the object or by a label or mark attached on the surface of the object. The light reflected by the object is changed into a parallel light by the lens 3 and is received by a photo sensing unit 6 after passing through the half mirror 2 and a color filter 5. The color filter 5 has a specific spectral characteristic of transparency corresponding to the desired hue or color of the object sought so as to allow the light of the color of the object sought to pass through the filter. The output signal of the photo sensing unit 6 is amplified by an amplifier 7 and in turn applied to a comparator 8. The output signal of the amplifier 7 is compared with a reference voltage applied to the one input terminal of the comparator 8. The comparator produces a digital signal (e) that is a high level output (referred to as 1 hereinafter) or a low level output (referred to as 0 hereinafter) depending on whether or not the output level of the amplifier 7 exceeds the reference voltage. The contents of the digital signal (e) i.e., 1 or 0 represent whether or not the photo sensing unit 6 receives the reflected light, accordingly the content of the digital signal (e) represents the hue of the object.
FIG. 2 is another example of a conventional color sensor and the reference numerals 9 and 10 show optical fibers respectively. In FIG. 2 like parts in FIG. 1 are designated by the same reference numerals. In this conventional color sensor, a light emitting diode is used as the light source 1 and the light of the light source 1 is projected on the object 4 through the optical fiber 9. The light reflected by the object 4 is passed through the optical fiber 10 and received by the photo sensing unit 6.
According to the color sensor shown in FIG. 2, the light source is a light having the same color as the desired color so that the filter 5 provided in the color sensor in FIG. 1 can be omitted and the area of the light emitting port of the optical fiber can be minimized and furthermore the light emitting port can be placed at very near the object so that the radius of spot light incident to the object can be minimized, thereby reducing the total size of the color sensor per se as compared to the color sensor shown in FIG. 1 and enabling the recognition of the color of smaller sized objects.
However, the conventional color sensors as described above are not capable of recognizing three or more colors because the recognizable color in the conventional color sensors is limited by the color of the filter 5, or the color of the light emitting diode 1. For example, when a red color filter or a red light emitting diode is used, the color sensor can recognize only whether the color of the object is red or not.
One way of recognizing more than three colors of the objects is to provide an arrangement for interchanging a plurality of filters of various kinds of colors in place of the color filter 5 in FIG. 1. For example, assuming that objects each having one of at least four different color groups such as red, green, yellow and a group consisting of all other colors are to be classified, red objects are classified using a red filter, subsequently green objects are classified using the green filter. The same operation is performed using the yellow filter and the remaining color filter to classify the yellow objects and other color objects are left undetected so that the objects can be classified into four color groups.
This method, however, requires to the repetition of the identifying means for each of the colors corresponding to the number of the kinds of the interchangeable color filters. Accordingly it takes much time and work to classify the color of the objects, and it further requires updating the reference voltage of the comparator 8 each time the color filters are interchanged.
Another method of classifying the color of the objects having more that three colors is to provide a plurality of color sensors for each of the colors to be classified. For example, if there are four colors to be classified, four color sensors each having a different color filter are employed. This method is effective to decrease the time for classifying the objects into four colors by placing each of the color sensors along the line of flow of the objects. However, the number of color sensors used increases as the number of the colors to be classified increases. Furthermore, in order to recognize the various colors correctly, the respective color sensors must have a uniform sensing characteristics. If there is dispersion of the color sensing characteristics among the respective color sensors due to the difference of the brightness of the light sources, the difference of the color filtering characteristics and/or the difference of the sensitivity of the respective photo sensing units, setting of the reference voltage of the respective comparators of the color sensors is very difficult.